求教大数阶乘的字符串实现原理【javascript】

//大数阶乘
function f(n) {  
    var a = [1];  
    for (var i = 1; i <= n; i++) {  
        for (var j = 0, c = 0; j < a.length || c != 0; j++) {  
            var m = (j < a.length) ? (i * a[j] + c) : c;  
            a[j] = m % 10;  
            c = (m - a[j]) / 10;  
        }  
    }  
    return a.reverse().join("");  
}  
console.log(f(1000));